Optical signatures of circumstellar interaction in type IIP supernovae

We propose new diagnostics for circumstellar interaction in Type IIP supernovae ( SNe IIP) by the detection of high-velocity ( HV) absorption features in H alpha and He I 10830 angstrom lines during the photospheric stage. To demonstrate the method, we compute the ionization and excitation of H and He in supernova ejecta taking into account time-dependent effects and X-ray irradiation. We find that the interaction with a typical red supergiant wind should result in the enhanced excitation of the outer layers of unshocked ejecta and the emergence of corresponding HV absorption, i.e., a depression in the blue absorption wing of H alpha and a pronounced absorption of He I 10830 8 at a radial velocity of about - 10(4) km s(-1). We identify HV absorption in H alpha and He I 10830 angstrom lines of SN 1999em and in H alpha of SN 2004dj as being due to this effect. The derived mass-loss rate is close to 10(-6) M-circle dot yr(-1) for both supernovae, assuming a wind velocity 10 km s(-1). We argue that in addition to the HV absorption formed in the unshocked ejecta, spectra of SN 2004dj and SN 1999em show a HV notch feature that is formed in the cool dense shell ( CDS) modified by the Rayleigh-Taylor instability. The CDS results from both shock breakout and radiative cooling of gas that has passed through the reverse shock wave. The notch becomes dominant in the HV absorption during the late photospheric phase, greater than or similar to 60 days. The wind density deduced from the velocity of the CDS is consistent with the wind density found from the HV absorption produced by unshocked ejecta.